1. Field of the Invention
The invention relates to a liquid crystal compound, and more particularly to a liquid crystal compound and composition capable of reducing driving voltage and a liquid crystal display and photoelectric device comprising the compound and composition.
2. Description of the Related Art
The twisted nematic liquid crystal display (TN LCD) is especially suitable for flat panel display (FPD) applications. A TN LCD applied to a watch or a calculator only needs a tiny matrix fragment, for example, 8*8, but such conventional TN LCDs cannot be applied to the matrix fragment of thin film transistor (TFT) LCD, for example, a 1024*768. Although applications of TN LCDs have been enormously changed, however, its original advantages still have be maintained, such as light weight, small size and low power consumption. The structure and operation of the LCD driven by thin film transistors can be found in U.S. Pat. No. 4,345,249. The development of TFT LCDs with fast switching have gradually replaced the cathode ray tube (CRT) and in order to improve contrast, chromatism, power consumption and viewing angle of LCDs, other switch modes such as a vertical aligned (VA) mode or in-plane switch (IPS) mode, have also been gradually developed. Nowadays, CRTs have almost entirely been replaced by LCDs.
The liquid crystal formulation suitable for use in LCDs preferably forms a liquid crystal phase between −40° C. and 100° C. and the clearing point thereof is preferably, at least 10° C. higher than the operation temperature. The actuation of liquid crystal molecules in a display is that, under an electric field, the direction of the liquid crystal molecules with dielectric anisotropy will be altered to a new direction which is vertical to the original liquid crystal direction after a voltage is applied thereto. For example, after applying a driving voltage, under an electric field, the axis direction of the positive nematic liquid crystal molecules commonly utilized in the TN mode or IPS mode will be parallel to the direction of the electric field. Contrary to the positive nematic liquid crystal molecules, the axis direction of the negative nematic liquid crystal molecules commonly utilized in the VA mode will be vertical to the direction of the electric field. According to the formula, Vth=π(K11/∈0Δ∈)1/2, the driving voltage of LCDs is affected by the polarity or elastic constant of a liquid crystal molecules, wherein, the higher the polarity (Δ∈) or the less the splay elastic constant (K11), the less the driving voltage. The liquid crystal formulation is formulated by liquid crystal monomers and the above-mentioned characteristics are all determined by the framework, substituent, molecular length and other construction characters of the liquid crystal molecules. The liquid crystal monomers with various weight percentages are formulated into appropriate liquid crystal formulations for an LCD.